1. Field of the Invention
The present invention relates to a method for producing a silicon single crystal by the Czochralski method, and more particularly, to a method for producing a silicon single crystal comprising the step of melting a raw material polycrystalline silicon in a crucible to produce a silicon melt for producing a silicon single crystal by the Czochralski method.
2. Discussion of the Background
When a silicon single crystal is produced by the Czochralski method, polycrystalline silicon is loaded into a crucible, melted to produce a desired quantity of silicon melt, and a single crystal is pulled from this silicon melt. This general process is well known to those of ordinary skill in the art, and is described in "Semiconductor Device Fundamentals", Crystal Growth, Robert. F. Pierret, pp. 16-18 (Addison-Wesley, 1996), as well as "Encyclopaedia of Chemical Technology", Crystal Growth, Kirk-Othmer, 4th ed., pp. 1092-1095 (John Wiley & Sons, 1997), both incorporated herein by reference.
Polycrystalline silicon used here as the raw material for producing a silicon single crystal is typically obtained by the Siemens method, which is considered excellent in production efficiency. The Siemens method is well known, and is described in "Encyclopaedia of Chemical Technology", High Purity Silicon Preparation, Kirk-Othmer, 4th ed., pp. 1090-1092 (John Wiley & Sons, 1997). The polycrystalline silicon rod is used as the raw material for producing a silicon single crystal, after the rod has been crushed by thermal shock or mechanical shock, etching, cleaning, and classifying.
More specifically, polycrystalline silicon lumps of the size between 45 and 85 mm, known as L-size, and polycrystalline silicon lumps of the size between 5 and 45 mm, known as S-size, are mainly used. Furthermore, small polycrystalline silicon pieces formed during the production of these polycrystalline silicon lumps, known as chips, or polycrystalline silicon granules produced by the fluidizing granulation method, are also used as the raw materials for producing silicon single crystals.
In melting the raw material for silicon single crystals, it is necessary to load as much of the raw materials into the crucible as possible, in order to produce a single crystal of as large a weight as possible.
To meet this requirement, increasing the size of the crucible has been considered. However, increasing the size of the crucible is not a preferred method, because if the size of the crucible is increased, the electric-power required for melting the raw materials will increase accordingly, and control of the melt temperature will become difficult.
Another method is to increase the height of the loading level of the raw materials in the crucible in order to increase the total quantity of raw materials loaded. For example, it is effective to stack the raw materials 2(lumps and/or granules); in the vicinity of the upper end of the crucible 1 as shown in FIG. 3(a). However, if the raw materials 2 are melted under these conditions, since the upper end of the crucible 1 is located far above the heater parts 4, the raw materials 2 situated in the vicinity of the upper end of the crucible 1 do not melt completely, and stick on the internal wall surface of the crucible 1 as shown in FIG. 3(b). As a result, as FIG. 3(c) shows, the upper end of the crucible 1 deforms inwardly, which causes a decrease in the life of the crucible 1 and the dislocation of the single crystal. Therefore, increasing the raw material level in the crucible is also not a preferred method.
In order to increase the quantity of the raw materials loaded into the crucible under such situations, the loading density must be increased. From this point of view, in actual operation, after L-size and S-size lump materials are loaded, chips or granular materials are often also loaded so as to fill the gaps between block materials. This loading method is advantageous from the point of view of raw material cost reduction through the effective use of the chips.
However, even such a loading method is not fully effective in increasing the amount of raw materials loaded. This is because many gaps remain between pieces of raw materials due to the large surface area of each piece of raw material. Therefore, the quantity of the polycrystalline silicon loaded into the crucible is still limited, and the production of a single crystal of large weight is difficult.